Engine systems include a variety of sensors for measuring ambient conditions, such as ambient temperature, pressure, and humidity. Based on the ambient conditions, one or more engine operating parameters may be adjusted to optimize engine performance. For example, based on ambient humidity (that is, the humidity in the air charge received in the engine), parameters such as an amount of exhaust gas recirculation (EGR), spark timing, and combustion air-fuel ratio may be adjusted.
Various kinds of sensors may be used to estimate the ambient humidity. As one example, oxygen sensors, such as a Universal Exhaust Gas Oxygen (UEGO) sensor used for exhaust air-fuel ratio control, may be used for ambient humidity estimation under selected conditions. Such oxygen sensors can be located in an exhaust passage or an intake air passage. In one example, shown by Surnilla et al. in US 20140202426, an exhaust gas oxygen sensor coupled to an engine bank may be utilized to opportunistically determine ambient humidity during conditions when the bank is selectively deactivated, and while the other bank continues to combust. A variable voltage may be applied to the sensor, and a change in pumping current may be correlated with the ambient humidity. In still other examples, humidity is estimated by a dedicated humidity sensor, such as shown by Surnilla et al. in US 20120227714. Therein the humidity sensor is positioned in an intake passage, downstream of an EGR valve.
The inventors herein have recognized potential issues with the above systems. As one example, humidity measurement conditions may be non-specific. In some cases, ambient humidity is estimated opportunistically, when possible. In still other cases, ambient humidity may be estimated when a humidity estimate is required. This can result in unnecessary humidity measurements. In the approach of Surnilla where an exhaust oxygen sensor is used for humidity sensing, the frequent application of a variable voltage to the sensor for the unnecessary humidity measurements can result in sensor blackening, and eventual degradation. In other cases, opportunistic humidity sensing can result in infrequent humidity estimation. Therein, engine operations may continue to be adjusted based on a most recent humidity estimate. However, in between the opportunistic humidity estimations, there may be changes in ambient conditions which affect the ambient humidity. For example, there may be a change in ambient temperature which affects the amount of water that an engine intake aircharge is able to contain, thereby changing the ambient humidity. Likewise, there may be a change in barometric pressure (e.g., due to a change in altitude of vehicle engine operation) that affects the ambient humidity. As such, if the ambient humidity estimate is not sensed or updated when there is a significant change in ambient conditions, engine performance may be degraded. For example, if there is a significant rise in ambient temperature since the most recent humidity sensing, ambient humidity may be over-estimated. This results in EGR being under-delivered, leading to increased NOx emissions and degraded fuel economy.
The inventors herein have identified an approach by which the issue described above may be at least partly addressed. One example method for a vehicle engine comprises: in response to a higher than threshold change in ambient air temperature or pressure, operating an oxygen sensor to update an ambient humidity estimate; and adjusting an engine actuator based on the updated ambient humidity estimate. In this way, humidity measurements may be triggered during specific conditions.
As an example, ambient conditions of a vehicle engine may be continually monitored and used to trigger humidity sensing. A change in the ambient temperature may include a change in the outside air temperature (OAT) as measured by an OAT sensor coupled to an outside of the vehicle. Alternatively, the change in ambient temperature may be inferred based upon a change in the air charge temperature (ACT) or intake air temperature (IAT) as measured by an IAT sensor coupled to an engine intake passage. A change in ambient temperature may be based on an absolute signal output by the above-mentioned temperature sensors, or a change (or derivative) in the output of the sensors. If a difference between the current ambient temperature and the ambient temperature at the last known humidity measurement is significant (e.g., higher than a threshold), it may be determined that ambient humidity may have changed significantly too, and. accordingly, a humidity measurement is triggered. Herein, humidity sensing may be performed via any one of an intake oxygen sensor, an exhaust gas oxygen sensor, and a (dedicated) humidity sensor. In the same way, a significant change in ambient pressure may be used to trigger a humidity measurement. Further still, ambient humidity sensing may be triggered if a threshold duration or distance of engine operation has elapsed since a last sensing.
In this way, changes in ambient conditions that may influence ambient air humidity may be monitored and used to trigger humidity sensing. By actively sensing humidity during conditions when humidity is expected to have significantly changed, rather than (or in addition to) opportunistically sensing humidity when possible, a more accurate and reliable humidity estimate may be provided for engine control. The technical effect of using a change in ambient temperature and pressure, along with other conditions, to trigger humidity measurement is that a humidity estimate can be updated when a substantial change in humidity is expected, reducing any unnecessary humidity measurements. In addition, in engine systems where an oxygen sensor is used for humidity estimation, the selective sensing of humidity when the specific trigger conditions are met decreases the chances of sensor degradation due to blackening from the applying of a variable voltage, and increases component life. By enabling a more reliable and up-to-date humidity estimate to be provided, engine operation can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.